Interpretive Summary: The need for more water use efficient crops is accelerating due to climate change and increased competition for water resources, yet apple cultivars are selected for fruit quality, disease and insect resistance, not water use efficiency. Measure of water use efficiency (WUE) is difficult on a seasonal basis for perennial woody crops like apple, however, simple measurements of plant ash and carbon isotope discrimination (delta13C) have been correlated with WUE in herbaceous plants. The purpose of this study was to measure WUE of mature and productive apple trees in the field and evaluate WUE correlations with ash and delta13C data to identify the most effective method. Both leaf ash and delta13C are suitable measurements of WUE in apple; leaf delta 13C is the most useful analysis but leaf ash would also be useful in identifying extremes in WUE at a much lower cost than delta13C with a higher through-put potential. Use of leaf ash or delta13C in apple breeding programs would lead to more water use efficient orchards in the future.

Technical Abstract:
Apple cultivars are selected for fruit quality, disease and insect resistance, not water use efficiency (WUE), however, the need for more water use efficient crops is accelerating due to climate change and increased competition for water resources. On a whole plant basis, calculation of water use efficiency (WUE) of perennial woody plants is a difficult endeavor requiring measurement of seasonal leaf, fruit and wood dry matter (DM) together with water use (WU), and expressed as DM/WU. WUE can also be calculated from short-term gas exchange measurements of photosynthesis (A) and transpiration (E) and expressed as A/E. Simple measurements of plant ash and carbon isotope discrimination have been correlated with water use efficiency in herbaceous plants. The purpose of this study was to measure WUE of mature and productive apple trees in the field and evaluate WUE correlations with ash and delta13C data to identify the most effective method. WUE was measured in whole tree gas exchange studies and from seasonal dry matter yield studies in ‘Empire’ apple for 2005, 2007 and 2009. In gas exchange studies leaf ash and delta13C were positively correlated with mean tree photosynthetic rate (A) and transpiration (E subc) and leaf delta13C was negatively correlated with mean tree WUE (A/E subc). In seasonal dry matter yield studies, leaf ash and delta13C responses were negatively correlated with actual (AWUE) and potential (PWUE) WUE. If leaf ash or delta13C are used as direct measurements of WUE, then leaf delta13C is the most useful analysis but leaf ash would also be useful in identifying extremes in WUE at a much lower cost with a high through-put potential.